A converter is a power processing circuit that may have an input-output isolation transformer and generally operates to convert an input voltage waveform with a DC component into an output DC voltage waveform. The presence of an isolation transformer often requires the use of a rectifier circuit in the converter output circuit to perform the waveform conversion. The traditional rectifier uses rectifying diodes that conduct the load current only when forward biased in response to the input waveform. In some rectifiers (i.e., synchronous rectifiers), the diodes are replaced by controllable switches that are periodically biased into conduction and nonconduction modes in synchronism with the periodic waveform to be rectified. In self-synchronized synchronous rectifiers, the biasing of the synchronous switches is supplied directly from the transformer to activate the synchronous switches. Further, in direct drive synchronous rectifiers, a drive signal is supplied from a separate source to the synchronous switches, while timing is provided from drive circuitry.
Converters are subject to abnormal conditions, such as turn-on and turn-off transients, as well as sudden changes in load and input voltage. When a converter uses synchronous-rectification, an additional precaution should be taken during these conditions since such converter can provide bi-directional power flow, and therefore bi-directional current flow through the converter. This means that the output of the converter can unintentionally become a power source and vice versa.
Turn-on transients become major concerns in systems where two or more converters employing synchronous rectification are connected in parallel without or-ing diodes or current unidirectional switches. In such cases, if proper control of synchronous rectifiers is not used, one of the converters could behave as a load, sinking current from the other converter, even at no load condition. Not only is such a system inefficient, but it could inhibit normal start-up during initial turn-on.
Turn-off transients are also important system concerns. If the synchronous rectifier, connected across the output, commonly through an inductor, is not disabled during this transition, a negative voltage at the output can occur due to resonance between an inductor and an output capacitor in a loop with the synchronous rectifier. Since it is a current bi-directional device, the synchronous rectifier allows negative inductor current flow that results in a negative output voltage, which, in most cases will destroy the load. This problem may also occur when two or more converters are connected in parallel.
Additionally, transients within the converter should be unloaded in such a fashion as to not damage the components therein or the load connected to the converter. If the load is suddenly removed from the converter, the energy stored in the inductor is suddenly discharged to the output capacitor. This discharge causes a voltage increase in the output capacitor and, consequently, on the output connectors. The increased voltage (commonly called a voltage overshoot) is proportional to the size of the inductor employed, the current through the inductor and size of the output capacitor.
As this voltage dramatically increases above its steady state value, the feedback loop (including a controller) disables the main switch via a driver and enables the synchronous rectifier so that the energy stored in the inductor continues to circulate through low resistance thus low dissipative synchronous rectifier. Consequently, most of the energy stored into the inductor is discharged into the output capacitor.
One such condition occurs when power stored in an output capacitor is discharged back into the converter, causing a negative current in the inductor. To combat this problem, various converters monitor the current in the inductor and when such current becomes negative, the synchronous rectifier is disabled to alleviate the problem. Unfortunately, such systems allow negative currents in the power converter which may cause damage to various components therein.
Accordingly, what is needed in the art is a system and method that provides improved response in view of transient conditions associated with a power converter.